Several methods are known for removing paint and generally such methods can be divided into three categories: mechanical, thermal and chemical.
Mechanical abrading processes are known in the art. Scraping is usually accomplished with either a putty knife or a paint scraper, or both. Putty knives range in width from one to six inches and have a beveled edge. A putty knife is used in a pushing motion going under the paint and working from an area of loose paint toward the edge where the paint is still firmly adhered. Paint scrapers are commonly available in 1 5/16, 2½, and 3½ inch widths and have replaceable blades. In addition, profiled scrapers can be made specifically for a particular purpose. A paint scraper is used in a pulling motion and works by raking the damaged areas of paint away. Scrapers may be formed from steel, aluminum, wood, Mycarta, and nylon, the desired choice being a softer material than the substrate being scraped to avoid marring for appearance or structural reasons.
Both the putty knife and paint scraper, particularly the paint scraper with its hooked edge, must be used with care to avoid “scribe scratches.” For example, when using metal scrapers to remove paint and sealant from aircraft skin, scribe scratching has been recognized as a significant problem, causing hairline cracks and fissures that can ultimately grow into larger cracks and threaten the structural integrity of the aircraft. The preferred methods for using scrapers on aircraft are discussed in the Aircraft Structural Repair Manuals (SRM) or FAA Advisory Circulars, AC 65-16A and AC 43.205. The following government documents document and discuss solutions to the issues of fatigue cracks subsequent to scribe mark scratches: The Federal Aviation Bulletin, FSAW 03-10B (amended), dated Mar. 31, 2004 “Fuselage Skin ‘Scribe Mark’ Damage on a Boeing 737 Aircraft; The Transport Canada Airworthiness Notice B071, Edition 1-3 Nov. 2004, “Paint and Sealant Removal Process.”
Other mechanical methods to remove paint include sanding, sandblasting, water blasting, plastic media blasting, and the like. All of these methods are characterized by modifying the surface being worked through surface abrasion, and in addition, they tend to be slow and labor intensive.
It is also known to use thermal methods, such as an electric heat plate and electric heat gun. Thermal methods have limited applications.
Finally, paint can be removed chemically. Solvent-base strippers have formulas that generally contain combinations of organic solvents such as methylene chloride, isopropanol, toluol, xylol, and methanol. Thickeners such as methyl cellulose and various additives such as paraffin wax are added to prevent the volatile solvents from evaporating before they have time to soak through multiple layers of paint. Some solvent-base strippers are quite thin and therefore unsuitable for use on vertical surfaces while others, called “semi-paste” strippers, are formulated for use on vertical surfaces or the underside of horizontal surfaces. More recently, paint removers have been formulated using a mixture of water and benzyl alcohol.
Chemical Paint Removers have several problems in their use. Paint removers, especially older formulations containing methylene chloride, are subject to extensive health and environmental restrictions. Since they are sprayed on the surface and allowed to work, applicator personnel must wear complete personal protective equipment while using the materials and the Volatile Organic Component (VOC) of the sprayed material is closely monitored in the U.S. Clean Air Act. Finally, at the conclusion of the stripping operation using chemicals, significant amounts of liquid/solid waste are generated and must be safely disposed.
A paint removal system that addresses some of the above-noted problems is desirable.